U.S. patent number 4,371,394 [Application Number 06/209,056] was granted by the patent office on 1983-02-01 for corrosion resistant austenitic alloy.
This patent grant is currently assigned to Carpenter Technology Corporation. Invention is credited to Terry A. DeBold, Michael Henthorne, Robert J. Yinger.
United States Patent |
4,371,394 |
Henthorne , et al. |
February 1, 1983 |
**Please see images for:
( Certificate of Correction ) ** |
Corrosion resistant austenitic alloy
Abstract
A corrosion resistant austenitic stainless steel alloy and
articles made therefrom consisting essentially in weight percent of
in which nitrogen ranges from a minimum of 0.15 w/o to no more than
the amount that can be retained in solid solution, the balance
being essentially iron, and the elements being balanced so that
cold rolled annealed specimens prepared with a crevice and tested
in accordance with ASTM G48-76 in 10 w/o FeCl.sub.3
.multidot.6H.sub.2 O at 50.degree. C. for 72 hours have a weight
loss of less than 0.3 gram. An embodiment that is particularly well
suited for making autogeneously welded articles, e.g. tubing, for
uses requiring exposure to brackish water consists essentially in
weight percent of: ______________________________________ (w/o)
______________________________________ C 0.06-0.08 Mn 4-6 Si 0.6
Max. Cr 20.5-21.5 Ni 14.5-15.5 Mo 4.8-5.4 B 0.0015-0.0035 Al 0.5
Max. N 0.20-0.25 ______________________________________ in which
the balance is essentially iron.
Inventors: |
Henthorne; Michael (Los Altos,
CA), Yinger; Robert J. (Jamul, CA), DeBold; Terry A.
(Wyomissing, PA) |
Assignee: |
Carpenter Technology
Corporation (Reading, PA)
|
Family
ID: |
22777141 |
Appl.
No.: |
06/209,056 |
Filed: |
November 21, 1980 |
Current U.S.
Class: |
420/46 |
Current CPC
Class: |
C22C
38/58 (20130101) |
Current International
Class: |
C22C
38/58 (20060101); C22C 038/58 () |
Field of
Search: |
;75/128A ;148/38 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Baumel et al., "New Austenitic Stainless Steels", Proceedings of
the 5th International Congress on Metallic Corrosion, NACE, 1974,
pp. 934-941..
|
Primary Examiner: Skiff; Peter K.
Attorney, Agent or Firm: Jay; Edgar N.
Claims
What is claimed is:
1. A corrosion resistant austenitic stainless steel alloy
consisting essentially in weight percent of about
in which nitrogen ranges from a minimum of 0.15 w/o to no more than
the amount that can be retained in solid solution, the minimum
amount of carbon plus nitrogen being equal to: ##EQU4## the balance
being essentially iron, and the elements being balanced so that
cold rolled annealed specimens prepared with a crevice and tested
in accordance with ASTM G48-76 in 10 w/o FeCl.sub.3.6H.sub.2 O at
50 C. for 72 hours have a weight loss of less than 0.3 gram.
2. The alloy set forth in claim 1 containing no more than about 5.4
w/o molybdenum.
3. The alloy set forth in claim 1 containing no more than about 7.5
w/o manganese.
4. The alloy set forth in claims 1, 2 or 3 containing no more than
0.6 w/o nitrogen.
5. The alloy set forth in claims 1, 2 or 3 containing at least 0.20
w/o nitrogen.
6. The alloy set forth in claim 5 containing no more than 0.5 w/o
nitrogen.
7. The alloy set forth in claim 5 containing no more than 0.08 w/o
carbon.
8. The alloy set forth in claim 7 containing no more than 0.07 w/o
aluminum, and no more than 0.005 w/o boron.
9. The alloy set forth in claim 7 containing no more than 0.05 w/o
aluminum, and 0.0015-0.0035 w/o boron.
10. The alloy set forth in claim 7 containing at least 0.06 w/o
carbon.
11. The alloy set forth in claim 10 containing no more than 0.03
w/o phosphorus and no more than 0.005 w/o sulfur.
12. The alloy set forth in claim 11 containing no more than 0.07
w/o aluminum.
13. The alloy set forth in claim 11 containing no more than 0.05
w/o aluminum.
14. The alloy set forth in claim 2 containing 0.06-0.08 w/o
carbon.
15. The alloy set forth in claim 14 containing 4-6 w/o manganese
and 0.20-0.25 w/o nitrogen.
16. The alloy set forth in claim 15 containing 20.5-21.5 w/o
chromium, and 14.5-15.5 w/o nickel.
17. The alloy set forth in claim 16 containing no more than about
0.005 w/o boron.
18. The alloy set forth in claim 16 containing 0.0015-0.0035 w/o
boron.
19. A corrosion resistant austenitic article which consists
essentially in weight percent of about:
in which nitrogen ranges from a minimum of 0.15 w/o to no more than
the amount that can be retained in solid solution, the minimum
amount of carbon plus nitrogen being equal to: ##EQU5## the balance
being essentially iron, and in which the elements are balanced so
that cold rolled annealed specimens thereof prepared with a crevice
and tested in accordance with ASTM G48-76 in 10 w/o
FeCl.sub.3.6H.sub.2 O at 50.degree. C. for 72 hours have a weight
loss of less tha 0.3 gram.
20. The article set forth in claim 19 containing no more than 5.4
w/o molybdenum.
21. The article set forth in claim 19 containing no more than 7.5
w/o manganese.
22. The article set forth in claim 19 containing no more than 5.4
w/o molybdenum, no more than 7.5 w/o manganese, and at least 0.20
w/o nitrogen.
23. The article set forth in claim 22 containing no more than 0.08
w/o carbon.
24. The article set forth in claim 23 containing at least 0.06 w/o
carbon.
25. The article set forth in claim 24 containing 4-6 w/o
manganese.
26. The article set forth in claim 25 which includes at least one
autogenous weld and which contains 20.5-21.5 w/o chromium,
14.5-15.5 w/o nickel, no more than 0.25 w/o nitrogen, and
0.0015-0.0035 w/o boron.
Description
This invention relates to corrosion resistant austenitic stainless
steel and articles made therefrom and more particularly to such
steel and articles made therefrom which are resistant to chloride
crevice and pitting corrosion.
Alloys of chromium, nickel and iron containing varying amounts of
molybdenum, manganese and nitrogen have hitherto been known which
provide a good combination of mechanical and chemical properties.
However, there has long been a particular need for an austenitic
stainless steel alloy having good mechanical properties and capable
of withstanding pitting and crevice corrosion in the presence of
chloride ions. Hitherto, alloys provided for making articles used
in chloride environments such as brackish water have left much to
be desired or when capable of providing a required degree of
corrosion resistance, particularly resistance to chloride pitting
and crevice attack (such as is measured by exposure to ferric
chloride, FeCl.sub.3, at 50.degree. C.) had been expensive to
produce and/or difficult to fabricate into the required
articles.
A. Baumel, E. Horn and H. Grafen* point out the difficulties
encountered in providing austenitic stainless steel articles
requiring pitting and crevice corrosion resistance in aggresive
media containing chlorine ions. They attribute such difficulties
with Cr-Ni-Mo stainless steel containing in weight percent (w/o)
nominally about 0.05 w/o Max. carbon, 17 w/o Cr, 13 w/o nickel, to
the presence of delta-ferrite and point out that an addition of
0.15 w/o nitrogen to a composition containing 0.03 w/o carbon Max.,
17 w/o Cr, 13 w/o Ni, 5 w/o Mo, the balance essentially iron
provides a homogeneously austenitic structure. The authors also
point out that stabilization of the austenitic balance by the
nitrogen addition prevents delta-ferrite from decomposing into
sigma phase during heat treatment or welding. While it is indicated
that a ferrite free, homogeneous austenitic structure could also be
obtained through an appropriate increase in the nickel-content,
nitrogen is credited with retarding precipitation of intermetallic
phases and carbides from the austenite. Baumel U.S. Pat. No.
3,726,668, Apr. 10, 1973 relates to a welding filler material
containing 0.001-0.2, preferably 0.001-0.1, w/o carbon, 0.1-5.0,
preferably 0.1-2.0, w/o silicon, 0.25-10.0, preferably 0.25-5.0,
w/o manganese, 15.0-25.0, preferably 15.0-20.0, w/o chromium,
3.5-6.0, preferably 3.5-5.0, w/o molybdenum, 8.0-30.0, preferably
10.0-16.0, w/o nickel, 0.01-3.0, preferably 0.01-1.5, w/o copper,
0.1-0.35, preferably 0.1-0.2, w/o nitrogen and the balance iron for
use in providing austenitic surface weld layers or welded joints on
predominantly austenitic substrate.
Deverell U.S. Pat. No. 4,007,038, Feb. 8, 1977, relates to Cr-Ni-Mo
austenitic stainless steel containing 14-21 w/o Cr, 20-40 w/o Ni,
6-12 w/o Mo plus up to 0.2 w/o C, up to 2 w/o Mn, 0.006 w/o or less
S, up to 1.00 w/o Nb, up to 0.5 w/o V, to which 0.005-0.05 w/o Ca
and 0.010-0.20 w/o Ce or a maximum of 0.07 w/o Ce+Ca are added for
the purpose of improving hot-workability as represented by the
degree of edge checking.
Chivinsky et al, U.S. Pat. No. 4,099,966, July 11, 1978, discloses
an austenitic stainless steel alloy described as being hot
workable, as having superior pitting and corrosion resistance to
the chloride ion and containing up to 0.1, preferably below 0.08,
w/o carbon, 2.5-15, preferably 8-13.5, w/o manganese, up to 1,
preferably below 0.75, w/o silicon, up to 0.01, preferably 0.007,
w/o Max. sulfur, 19-23, preferably 9.5-22, w/o chromium, 5-16,
preferably 9-13, w/o nickel, 3-5, preferably 3.5-4.5, w/o
molybdenum, up to 1 w/o niobium, up to 0.3 w/o vanadium, up to 0.3
w/o titanium, nitrogen from 0.2 w/o to the limit of its solubility,
preferably 0.23-0.33 w/o nitrogen, up to 0.1 w/o of cerium, calcium
and magnesium combined up to 3 w/o copper and the balance iron.
The U.S. Pat. No. 4,007,038 and the U.S. Pat. No. 4,099,966
reinforce the views expressed by Baumel, Horn and Grafen with
regard to difficulties with Cr-Ni-Mo austenitic stainless steel
alloys designed to resist chloride ion attack.
The present invention stems from the discovery that when the
elements chromium, nickel and molybdenum are maintained within
critically narrow limits, and the elements carbon, nitrogen and
manganese are balanced in relation to each other and to the
elements chromium, nickel and molybdenum, an austenitic stainless
steel is provided characterized by outstanding resistance to
chloride cervice and pitting corrosion. The alloy is suitable for a
wide variety of uses depending upon how the elements, particularly
manganese and nitrogen, are balanced within their stated ranges.
For example, when the elements manganese and nitrogen are kept
within their stated ranges but below sharply critical levels, the
alloy provided is especially suited for autogenous welding and
provides articles, for example welded tubing having outstanding
resistance to chloride crevice and pitting corrosion. The
combination of strength and corrosion resistance provided with the
higher levels of nitrogen contemplated herein make the composition
highly advantageous for use in such demanding areas as surgical
implants or stranded cable for subsurface use in the ocean. The
composition affords a desirable degree of flexibility in that its
high strength makes it possible to decrease the amount of working
or the amount of material required to attain a given strength level
or load carrying capability.
It is, therefore, a principal object of this invention to provide a
Cr-Ni-Mn-Mo-N austenitic stainless steel and products made
therefrom having good resistance to chloride crevice and pitting
corrosion which steel lends itself to production and working by
conventional techniques.
Another object is to provide articles intended for use requiring
exposure to chloride ions, particularly articles such as
autogenously welded tubing exposed in use to brackish water,
characterized by outstanding resistance to pitting and crevice
corrosion.
The foregoing as well as additional objects and advantages are
attained by carefully balancing the composition which consists
essentially of the broad and preferred amounts in weight percent
(w/o) of the elements indicated in Table I, the balance being iron.
However, it is to be noted that the preferred minimum or maximum
amount of one or more elements can be used with the broad maximum
or minimum amounts respectively of the remaining elements to form
intermediate ranges or to adjust the composition properties as will
be more fully pointed out hereinbelow.
TABLE I ______________________________________ Preferred For
Autogenous Broad (w/o) Preferred (w/o) Welding (w/o)
______________________________________ C 0.03-0.1 0.03-0.08
0.06-0.08 Mn 4-11 4-7.5 4-6 Cr 20-23 20-23 20.5-21.5 Ni 14-18 14-18
14.5-15.5 Mo 4.8-5.6 4.8-5.4 4.8-5.4 N 0.15-0.6 0.20-0.5 0.20-0.25
C + N 0.23 Min. 0.23 Min. B 0.01 Max. 0.005 Max. 0.0015-0.0035
______________________________________
The balance of the composition is essentially iron which is
intended to exclude all further additions in amounts which
significantly alter the properties of the composition. For example,
depending upon which deoxydizing practice is followed, small
amounts of the elements used may be retained in the composition.
Thus, when silicon is used as a deoxydizer some will be retained in
the composition but should be limited, preferably to less than
about 0.6 w/o, because silicon may adversely affect intergrannular
corrosion resistance. Also, when present in too large an amount,
silicon may result in the presence of unwanted sigma phase or
ferrite. Aluminum may also be used as a deoxydizer but no more than
0.1 w/o, preferably no more than 0.07 or, better yet, no more than
0.05 w/o should be retained, because aluminum may tend to tie up
nitrogen. Aluminum is also a strong ferrite former and in too large
an amount may also objectionably detract from the hot workability
of this composition. Misch metal, which is a mixture of rare earths
made up primarily of cerium and lanthanum, can also be used for its
scavenging properties and beneficial effect on hot workability. To
that end, boron and misch metal can both be used. The beneficial
effect of misch metal, when it is used, does not require that any
definite amount of misch metal be retained in the composition and
preferably there is little or none; its beneficial effect being
provided during the melting process when, if used, up to about 0.4
w/o may be added.
Boron can be present in an amount up to about 0.005 w/o or even up
to 0.01 w/o because of its beneficial effect on the forgeability of
this composition. Because boron is believed to contribute to the
corrosion resistance of the composition, preferably about
0.0015-0.0035 w/o is present.
Such elements as phosphorus and sulfur are kept low. Preferably
phosphorus is limited to no more than 0.03 w/o and sulfur to no
more than 0.005 w/o.
In this composition, the elements chromium, nickel and molybdenum
are carefully balanced within the stated ranges in relation to each
other and the elements carbon, manganese and nitrogen to provide a
unique combination of mechanical and corrosion resistance
properties, especially chloride crevice and pitting corrosion
resistance. Of particular significance is that when the crevice
corrosion resistance of the worked and annealed composition of the
present invention is tested in accordance with ASTM G48-76, the
weight loss measured after exposure to 6 w/o ferric chloride at 50
C. for 72 hours is less than 0.3 grams. To ensure the attainment of
those properties, a minimum of about 20 w/o chromium, about 4.8 w/o
molybdenum and about 14 w/o nickel are required. When chromium
exceeds about 23 w/o, it contributes to the formation of second
phases as also does molybdenum in amounts in excess of about 5.6
w/o, and the presence of second phases is to be avoided because of
the adverse effect on corrosion resistance. Nickel works to ensure
an austenitic structure in the alloy of this invention and its
desired corrosion resistance. However, further additions of nickel
above about 18 w/o, though tolerable, add to the cost of the alloy
without correspondingly contributing to its usefulness. Best
results are attained when the larger amounts of chromium and
molybdenum are balanced with the larger amounts of nickel.
Preferably about 20.5-21.5 w/o chromium and about 14.5-15.5 w/o
nickel are used.
Carbon and, more importantly, nitrogen work together with nickel to
ensure the austenitic balance of this composition and to minimize,
preferably avoid entirely, the formation of phases which adversely
affect the desired properties, particularly corrosion resistance.
To that end, a minimum of about 0.03 w/o carbon and about 0.15 w/o
nitrogen is required in this composition. Excessive carbon tends to
adversely affect intergrannular corrosion resistance, probably
because of the formation of harmful amounts of carbides or
carbonitrides. For that reason, carbon is limited to no more than
about 0.1 w/o, preferably to no more than about 0.08 w/o. On the
other hand, nitrogen to the extent it can be retained in solution
can be used in much larger proportions than carbon to maintain the
austenitic structure of this composition and prevent the formation
of unwanted phases. Thus, up to about 0.6 w/o nitrogen or more can
be present.
Manganese works to increase the solubility of nitrogen in this
composition and is added to ensure the retention of nitrogen in
solution despite the fact that some of the nitrogen is required to
offset the otherwise adverse effect of manganese on the corrosion
properties of this composition. The adverse effect of manganese on
corrosion resistance appears to be greater with the larger amounts
of molybdenum contemplated herein with the result that more
nitrogen is required to counterbalance a given amount of manganese
when about 5.5 w/o molybdenum is present as compared to when about
5 w/o molybdenum is present.
With the other elements balanced as indicated in the broad range of
Table I, when molybdenum is increased over its range from 4.8-5.6
w/o, relatively small changes in the molybdenum content have a
substantial, adverse effect on chloride pitting and crevice
corrosion resistance. That effect can be offset by an increase in
the carbon plus nitrogen content of the composition. In view of the
fact that no more than about 0.1 w/o carbon, preferably no more
than about 0.08 w/o, is present, the amount of nitrogen present is
increased together with the manganese as required to ensure that
the nitrogen is retained in solution. In practice, it has been
found that when the amounts of other elements present are
substantially unchanged, and the amount of molybdenum present is
increased by a few tenths of a percent, a useful guide in
determining the corresponding minimum increase in the amount of
nitrogen required to counter the adverse effect of the increase in
molybdenum is about one tenth of the amount by which the molybdenum
content has been increased. That is: ##EQU1## If it should prove to
be necessary to increase the amount of manganese present in order
to ensure retention of the increased amount of nitrogen in
solution, then a somewhat larger increase, that is, several
hundredths of a percent in the nitrogen content is preferred. The
precision by which the amount of molybdenum and nitrogen present in
this composition can be routinely determined varies about plus or
minus 0.08% in the case of molybdenum and about plus or minus 0.01%
to about 0.03% over the nitrogen range contemplated herein.
However, when special pains are taken, that precision can be
improved. In the case of the nitrogen determination, the analytical
tolerance can be reduced to as little as plus or minus 0.005% at
the low end of the nitrogen range and to as little as plus or minus
0.015% at the upper end. As a guide in adjusting the nitrogen
content with molybdenum present in an amount equal to about 5.5 w/o
it has been observed that with a manganese content of about 4 w/o,
the carbon plus nitrogen content should preferably be at least
about 0.3 w/o, with about 6 w/o manganese, the carbon plus nitrogen
content should preferably be about 0.35 w/o, at about 8 w/o
manganese, the carbon plus nitrogen content should be at least
about 0.4 w/o, at about 9 w/o manganese, the carbon plus nitrogen
should preferably be at least about 0.45 w/o, and at about 11 w/o
manganese, carbon plus nitrogen should be at least about 0.5 w/o.
That is: ##EQU2## and combining Equations 1 and 2 gives:
##EQU3##
This composition is melted, cast and worked using well-known
metallurgical techniques. Preferably, deoxydation of the heats is
carried out using boron with aluminum and/or silicon. When forging
is to be carried out, it is preferably done from a furnace
temperature of about 2100 F. (1150-1200 C.). Annealing is
preferably carried out at about 2150 F. (1175 C.).
The following examples of the present invention having the
composition indicated in Table II were prepared as small,
experimental heats and cast as ingots which were forged and hot
rolled from a furnace temperature of 2100 F. (1150 C.), annealed in
air at 2150 F. (1175 C.) for one half hour, cold rolled to 0.125
inch (0.32 cm) strip, annealed, and cut to form the required
specimens for testing in accordance with ASTM G48-76.
TABLE II ______________________________________ Ex. No. C Mn Cr Ni
Mo N ______________________________________ 1 .068 7.49 20.89 15.19
5.46 .32 2 .074 4.02 20.89 15.27 5.51 .32 3 .076 6.08 21.27 15.23
5.52 .30 4 .072 11.35 21.24 15.40 5.55 .57 5 .069 9.35 21.07 15.33
5.46 .40 6 .072 7.54 21.13 15.25 4.93 .26 7 .063 5.21 21.37 15.27
4.92 .19 8 .073 5.14 21.27 15.15 4.99 .29 9 .069 7.18 21.13 15.70
5.07 .38 10 .073 4.97 20.95 15.69 5.05 .42 11 .072 7.38 20.92 15.30
4.98 .35 12 .073 5.05 21.08 15.17 4.94 .31 13 .081 7.52 20.97 14.96
4.98 .27 14 .076 7.91 21.21 15.25 5.55 .42
______________________________________
In each instance the balance was iron except for small amounts of
but less than 0.6 w/o silicon, less than 0.03 w/o phosphorus, less
than 0.005 w/o sulfur except Examples 1 and 4 contained 0.006 w/o
sulfur, about 0.002-0.004 w/o boron except that Example 2 contained
less than 0.0005 w/o boron, and each contained about 0.02-0.04 w/o
cerium plus lanthanum except Example 4 which contained only 0.003
w/o and Example 7 which contained 0.055 Ce+La.
Unless otherwise indicated duplicate test specimens were prepared
and tested in accordance with ASTM G48-76. Cold rolled specimens
which had been annealed at 2150 F. (1176 C.) for 12 minutes and
then air cooled (CRA) were subjected to the crevice test in 10 w/o
FeCl.sub.3.6H.sub.2 O at 50.degree. C. for 72 hours. The specimens
were weighed prior to and after exposure to the test environment to
determine the weight loss in grams. A chloride pitting corrosion
test without a crevice was also carried out in accordance with ASTM
G48-76 on three sets of specimens. One set was made up of welded
specimens which had not been annealed and two sets were welded and
annealed with two different annealing treatments. The welded
specimens were first cold rolled and annealed and then gas tungsten
arc welded. One third of the welded specimens was not annealed,
another third was annealed for 35 seconds at 2150 F. in molten salt
and then quenched in water (W+Ann, WQ) and the final third was
annealed at 2150 F. for 12 minutes and then cooled in air (W+Ann,
AC). The weight loss suffered by each specimen in grams is set
forth in Table III.
TABLE III ______________________________________ (Weight Loss in
Grams) Crevice Ex. Test Pitting Test No. CRA Welded W + Ann, WQ W +
Ann, AC ______________________________________ 1 .2390 .8122 .0004
.0869 .1712 .8802 -- .0918 2 .0125 .5127 0 0 .1614 .9046 0 0 3
.1975 .8135 .0002 .0009 .1045 .9048 0 .0008 4 .0003 .0001 .0002
.0002 0 .0701 .0002 0 5 .2357 1.2024 .0016 .0135 .2622 1.2809 .0002
.0016 6 .1641 .7064 .0617 0 .2254 .8009 .0016 0 7 .2179 .6221 .0005
1.0276 .2547 1.1222 .5020 .3647 8 .0166 .3180 .0007 0 .0090 .0946
.0002 0 9 .0031 .0005 .0008 .0044 .0030 0 .0027 0 10 .0012 0 .0006
0 .0091 0 .0005 0 11 .1197 .0899 .0010 .0016 .0954 .0004 .0234
.0024 12 .0013 .0400 .0959 .0003 .0122 .1645 .0009 .0005 13 .1482
.0009 -- -- .0145 .0008 -- -- 14 .0071 -- -- -- .0003 -- -- --
______________________________________
Specimens of Examples 1-14 in the cold rolled annealed condition
(CRA) showed no harmful effect of sigma phase. In the case of the
as welded specimens, sigma phase was found in all specimens except
for the specimens of Examples 4, 9 and 10. The welded and annealed
specimens of Examples 1-6 and 8-13 were free of the harmful effects
of sigma. Example 7 demonstrates the less than preferred chloride
corrosion resistance with the relatively low nitrogen content of
0.19 w/o. Longer annealing time, e.g. up to about one-half hour,
followed by quenching in water should be used when better welded
plus annealed corrosion properties are wanted.
The following heats having the composition indicated in Table IV
were prepared as was described in connection with Examples
1-14.
TABLE IV ______________________________________ Heat C Mn Cr Ni Mo
B N Ce + La ______________________________________ A .076 7.38
21.20 15.03 5.45 .0031 .25 .022 B .072 11.26 21.45 15.38 5.49 .0031
.35 .019 C .072 7.37 20.96 15.06 4.99 .0025 .17 .053 D .071 7.37
17.37 15.34 5.45 .0032 .33 .021 E .066 7.49 19.49 15.27 5.46 .0033
.35 .024 F .073 7.58 21.25 15.23 4.50 .0030 .34 .020 G .065 7.48
20.94 15.21 6.49 .0029 .37 .028
______________________________________
As in the case of Examples 1-14, the balance of each heat was iron
except for less than about 0.6 w/o silicon, less than 0.03 w/o
phosphorus, and less than 0.005 w/o sulfur.
Duplicate test specimens of each of Heats A-G were prepared as
described in connection with Examples 1-14 and tested in accordance
with ASTM G48-76. The results of the crevice corrosion and pitting
tests are set forth in Table V.
TABLE V ______________________________________ (Weight Loss in
Grams) Crevice Pitting Test Heat CRA Welded W + Ann, WQ W + Ann, AC
______________________________________ A .6037 1.4061 .0398 1.0530
.5877 1.1940 .0005 1.2325 B 1.0880 1.3173 0 .8091 1.0791 1.2085
.3551 .6750 C .2400 .8961 .5067 1.3551 .2826 .9871 .2009 1.2256 D
.9959 .9956 1.0107 .4869 1.0361 1.0362 1.1444 .5152 E .5293 .6068
.1115 .0007 .3442 .6588 .3312 .0003 F .4230 .0770 .0078 .0105 .4114
.5880 .1685 .0387 G 1.2708 .0008 .0002 .3511 1.3449 .5030 .3386
.5118 ______________________________________
Most, if not all, of the test material was examined with the
optical microscope to confirm the presence of sigma phase in the
cold rolled specimens that showed excess weight loss in the crevice
corrosion test. Specimens of Heats A and B cold rolled annealed
(CRA) were also examined with the scanning electron microscope.
Both the cold rolled annealed and welded specimens of Heats A and B
showed sigma phase. Heat A demonstrates the criticality of
increasing the nitrogen content sufficiently when the manganese
content is 7.38 w/o as compared to Example 3 with 6.08 w/o
manganese. When 7.49 w/o manganese is balanced with 0.32 nitrogen
(0.338 w/o C+N) as in Example 1 chloride crevice corrosion
resistance is improved. The poor chloride corrosion resistance of
Heat B is to be contrasted with the outstanding corrosion
resistance of Example 4 where 11.35 w/o manganese was balanced with
0.57 w/o nitrogen (0.642 w/o C+N). Heat C demonstrates that even
with molybdenum reduced to 4.99 w/o, 0.17 w/o nitrogen (0.242 w/o
C+ N) is not enough to balance 7.37 w/o manganese and provide good
chloride pitting corrosion resistance in the as welded and annealed
condition. Heats D and E are believed to demonstrate the adverse
effect when chromium is too low and Heats F and G demonstrate
respectively the effect on chloride crevice corrosion resistance
when the composition contains too little or too much
molybdenum.
In accordance with another embodiment of this invention, the
elements C, Mn, Cr, Ni, Mo, N and B are balanced as indicated in
the right-hand column of Table I to provide an alloy which not only
has a high degree of resistance to chloride crevice and pitting
corrosion resistance, but which is particularly suited for
autogenous welding to provide welded products characterized by
outstanding resistance to chloride crevice and pitting corrosion.
The following example is illustrative of this embodiment.
EXAMPLE 15
As a further example of this composition, a heat was melted and
cast into ingots containing:
______________________________________ w/o
______________________________________ Carbon 0.07 Manganese 5.36
Silicon 0.28 Phosphorus 0.021 Sulfur 0.007 Chromium 20.41 Nickel
15.39 Molybdenum 5.06 Nitrogen 0.25 Boron 0.0038
______________________________________
The balance was iron and incidental amounts of other elements.
Forging and hot rolling to 0.220 in (0.56 cm) strip were carried
out from a temperature of 2150.degree.-2200.degree. F.
(1175.degree.-1200.degree. C.). The thus formed strip was annealed,
cleaned and then cold rolled to 0.028 in (0.071 cm) strip. The
cold-rolled strip was annealed and formed into test specimens in
accordance with the specifications of the appropriate ASTM test.
When tested in that condition, the 0.2 percent yield strength was
56,000 psi (386.1 MPa), the tensile strength was 113,000 psi (779.1
MPa), the elongation in 2 inches (5.08 cm) was 45.0 percent. The
hardness in that condition was Rockwell B85.
Duplicate chloride corrosion test specimens were prepared as
described and then tested in accordance with ASTM G48-76 in
FeCl.sub.3 at 50.degree. C. for 72 hours. In addition to flat
specimens, lengths of tubing formed by autogenously welding and
annealing previously described strip were also tested. The
duplicate welded and annealed specimens, when tested for pitting,
one had no weight loss and the other had a weight loss of 0.0022
gram. In the case of duplicate flat specimens tested with crevices,
one had a weight loss of 0.1154 g, and the other a weight loss of
0.0476 g. When for purposes of comparison, an alloy of the U.S.
Pat. No. 4,007,038 (containing 0.025 w/o C, 1.6 w/o Mn, 20 w/o Cr,
24.5 w/o Ni, 6.4 w/o Mo, 0.032 w/o N, 0.0012 w/o B and balance
iron) was subjected to the same test for crevice corrosion, one
duplicate specimen had a weight loss of 0.4240 g, and the other had
a weight loss of 0.9098 g.
For further comparison with Example 15, Heat H was prepared as
described in connection with Example 15 having the following
composition, the balance being iron:
______________________________________ Heat C Mn Si P S Cr Ni Mo N
B ______________________________________ H .07 5.37 .27 .022 .006
20.52 15.38 5.02 .27 .0027
______________________________________
The only significant difference between Example 15 and Heat H is
believed to be the larger average nitrogen content of 0.27. When
coils of the alloy of Example 15 and of Heat H were autogenously
welded into 11/8 inch (2.86 cm) OD tubing having a wall thickness
of 0.028 in (0.071 cm) problems were encountered with the material
formed from Heat H that did not occur with the Example 15 tubing.
During the welding of the Heat H coil, the arc was unstable, there
was considerable sparking and what was considered excessive
electrode erosion. This resulted from the small but significant
increase in nitrogen content. The Example 15 material was
autogenously welded under the same conditions without experiencing
those or any other significant difficulties. The mechanical
properties of Heat H as measured by room temperature tensile tests
did not differ significantly from the properties of the composition
of Example 15. The 0.2 percent yield strength of the specimens
formed from Heat H was 58,000 psi (399.9 MPa), the tensile strength
was 114,000 psi (786 MPa), and the elongation in 2 inches (5.08 cm)
was 41 percent.
The terms and expressions which have been employed are used as
terms of description and not of limitation, and there is no
intention in the use of such terms and expressions of excluding any
equivalents of the features shown and described or portions
thereof, but it is recognized that various modifications are
possible within the scope of the invention claimed.
* * * * *